1. Field of the Invention
The disclosures herein generally relate to a surface-emitting laser device, an optical scanner device and an image forming apparatus, and more particularly to a surface-emitting laser device capable of oscillating laser light in a direction orthogonal to a substrate, an optical scanner device having such a surface-emitting laser device, and an image forming apparatus having such an optical scanner device.
2. Description of the Related Art
A vertical cavity surface-emitting laser (VCSEL) device is a semiconductor laser device that oscillates laser light in a direction orthogonal to a substrate. The VCSEL generally has features such as (1) a low price, (2) low power consumption, (3) high performance with a smaller size and (4) easy to integrate two-dimensionally compared to an edge emitting laser (EEL) that oscillates laser in a direction parallel to a substrate.
The surface-emitting laser device (VCSEL) includes a confinement structure to enhance electric current injecting efficiency. As an example of such a confinement structure, a confinement structure obtained by selectively oxidizing aluminum-arsenic (AlAs) (hereinafter also called an “oxide confinement structure” for convenience) is frequently used.
The oxide confinement structure is obtained by forming a mesa of a predetermined size having a layer subject to selective oxidation (hereinafter also called a “selective oxidation layer”) that is formed of a p-AlAs layer exposed from sides of the mesa, and subsequently placing the mesa under a high-temperature water-vapor atmosphere to selectively oxidize Al from sides of the mesa, thereby causing an unoxidized region to remain in the selectively oxidized p-AlAs layer (i.e., the selective oxidation layer) near the center of the mesa. This unoxidized region corresponds to a drive current passing region (current injecting region) of a surface-emitting layer device. Thus, it may be easy to confine the electric current.
The refractive index of the oxidized Al (AlxOy) layer in the oxide confinement structure is approximately 1.6, which is lower than the refractive index of a semiconductor layer. Hence, the refractive index difference is formed in a transverse direction within a resonator structure to confine the laser light in the center of the mesa, which may eventually improve luminous efficiency of the laser light. As a result, the surface-emitting layer (VCSEL) device may be capable of implementing excellent properties such as a low threshold current and high luminous efficiency.
Examples of an applied field of the VCSELs include a light source for an optical recording system in a printer (oscillation wavelength: 780 nm band), a light source for recording in an optical disk device (oscillation wavelengths: 780 nm band, 850 nm band), and a light source for an optical transmission system such as a local area network (LAN) utilizing optical fibers (oscillation wavelengths: 1.3 μm band and 1.5 μm band). Further, the VCSELs may also be applied as a light source for optical transmission between boards, within a board, between chips in a large-scale integrated circuit (LSI), or within the chip of the integrated circuit.
In the aforementioned examples of the applied field of the VCSELs, light emitted from the VCSEL (hereinafter also simply called “emission light”) may preferably be directed in a certain polarization direction and preferably have a circular cross-section, and preferably be capable of emitting light orthogonal to a reference plane.
The prospective method for adjusting a polarization direction at present may be the VCSEL that employs an inclined substrate. Employing the inclined substrate in the VCSEL may make a crystal structure asymmetric relative to a main surface of the substrate. This may introduce anisotropy into optical gain. As a result, it may be possible to align the polarization in a specific direction in which the optical gain increases.
For example, Japanese Patent No. 4010095 (hereinafter referred to as “Patent Document 1”) discloses a surface-emitting semiconductor laser having a relatively simple configuration that is capable of controlling polarization of laser light in a certain direction, and capable of oscillating laser light with low threshold current to exhibit high output. The surface-emitting semiconductor laser disclosed in Patent Document 1 includes a main surface of a semiconductor substrate that is crystallographically inclined at an angle range of 15 to +5 degrees in a [1 1 0] direction based on a [1 0 0] direction relative to a surface having a crystal face orientation equivalent to a [1 0 0] plane and includes an active layer formed of GaAs/AlGaAs semiconductor. The disclosed surface-emitting semiconductor laser further includes a selective oxidation layer obtained by oxidizing, from its peripheral part, a macroscopically smooth layer having a cross sectional outer circumferential shape without singularity when cut in parallel with the main surface of the semiconductor substrate.
Further, Japanese Laid-open Patent Publication No. 2010-153768 (hereinafter referred to as “Patent Document 2”) discloses a surface-emitting laser device capable of exhibiting a stable polarization direction while controlling oscillation of a high-order transverse mode. The surface-emitting laser device disclosed in Patent Document 2 includes a p-side electrode formed around an emission region of an emission surface emitting laser light, and a transparent dielectric film formed in a peripheral region within the emission region to lower reflectivity of the peripheral region less than reflectivity of a central part of the emission region. In the surface-emitting laser device having the above configuration, the region having the low reflectivity within the emission region has anisotropy in two mutually orthogonal directions.
In addition, Japanese Patent No. 3566902 (herein after referred to as “Patent Document 3”) discloses a surface-emitting laser device having a transparent layer relative to an oscillation wavelength of an oscillation laser formed by coating a part of an internal surface of an opening of an upper electrode. In the surface-emitting laser device, the thickness of the transparent layer is (2i+1)/4n times (n represents a refractive index of the transparent layer, i represents an integer) of the oscillation wavelength of the oscillation laser.
Moreover, Japanese Laid-open Patent Publication No. 2011-009693 (hereinafter referred to as “Patent Document 4”) discloses a method for fabricating a surface-emitting laser device. The method includes layering a transparent dielectric layer on an upper surface of a layered product before forming of a mesa structure, forming a first resist pattern including a pattern regulating an outer shape of the mesa structure on the upper surface of the dielectric film and a pattern protecting a region corresponding to one of a high reflective region and a low reflective region of an emission region, etching the dielectric layer utilizing the first resist pattern as an etching mask, and forming a second resist pattern protecting a region corresponding to the entire emission region.